1. Field of the Invention
The present invention is generally in the field of semiconductor manufacturing. More specifically, the invention is in the field of semiconductor die packaging.
2. Background Art
Leadframe based molded packages are commonly used in semiconductor packaging. FIG. 1A shows a top view of leadframe 100. Paddle 102 of leadframe 100 is attached to the remainder of leadframe 100 by four tie bars 140, 142, 144, and 146. There is one tie bar at each corner of paddle 102. Tie bars 140, 142, 144, and 146 are attached to dam bar 101 of leadframe 100. A number of inner leads 112 are situated on all four sides of leadframe 100. The top surface of paddle 102 is silver-plated copper while the bottom surface of paddle 102 is unplated copper. Tie bars 140, 142, 144, and 146, A and inner leads 112 are also copper with silver plating on the top side where bond wires will be attached.
Semiconductor die 114 is attached to the top surface of paddle 102 on leadframe 100. Bond wires (not shown in any of the Figures) are bonded between semiconductor die 114 and leadframe 100. These bond wires have a first end bonded to a respective bonding pad. An example of a bonding pad is pointed to as bonding pad 116 in FIG. 1A. A second end of each bond wire is bonded to a respective inner lead 112. Bonding pads 116 can be aluminum while the bond wires can be gold or aluminum.
In FIG. 1A, leadframe 100 has not yet been through the molding process, thus leadframe 100 remains unencapsulated in mold compound. In a subsequent stage of the fabrication process, leadframe 100 is placed in a mold cavity in a molding machine and the entire die, bond wires, bonding pads, and paddle are encapsulated in a mold compound, leaving the outer leads of the package (not shown) exposed for soldering to the printed circuit board.
By way of background, in many RF applications where a leadframe based molded package is used, there is a need for a low inductance on the ground connection to the semiconductor die since the performance of various devices on the die can be significantly affected by inductance on the ground connection. One cause of high inductance on the ground connection is the relatively long path used to connect the ground of the semiconductor die to the ground of the printed circuit board. This long path includes, at a minimum, the length of a bond wire and a lead to provide a connection between the semiconductor ground and the printed circuit board ground. The bond wire utilized to make the ground connection could have an inductance of from one to three nano henrys. In addition, the leads on the leadframe could have a length of several millimeters and could result in an additional inductance of from three to six nano henrys. Thus, using the method discussed above results in an undesired inductance of from four to nine nano henrys on the ground connection of the semiconductor die.
In an effort to reduce the inductance values on the ground connection of the semiconductor die, an xe2x80x9cexposed paddlexe2x80x9d leadframe based molded package was developed. In an exposed paddle leadframe based molded package, the bottom surface of the paddle is not encapsulated in mold compound. The metal on the bottom surface of the paddle is left exposed in order to electrically connect the bottom surface of the paddle to the printed circuit board ground during a re-flow solder process. An exposed paddle leadframe is also referred to as a xe2x80x9cdeep downsetxe2x80x9d leadframe since the leadframe paddle resides in a plane below the rest of the leadframe. In other words, the paddle is set lower so that the bottom surface of the paddle would not be covered by the mold compound to be applied to the leadframe in a subsequent step. As such, the bottom surface of the paddle would remain exposed for soldering to the printed circuit board ground.
In an exposed paddle leadframe based molded package, the ground carrying bond wire is bonded to the ground pad on the semiconductor die. However, in contrast to an xe2x80x9cunexposed paddlexe2x80x9d leadframe based molded package, in an exposed paddle leadframe based molded package the second end of the bond wire is bonded to the grounded paddle of the leadframe instead of being bonded to a lead of the leadframe. By utilizing this method, the longer path of a lead and a bond wire for ground connection is replaced with the shorter path of a bond wire without the need to go through the lead. Moreover, the length of the bond wire itself is also shorter since the bond wire does not have to reach out to a lead, and instead connects to the grounded paddle. The inductance on these shorter bond wires can be as little as 0.3 to 0.5 nano henries. As stated above, the exposed bottom side of the paddle of the leadframe is soldered to the ground of the printed circuit board.
Thus, by using the paddle to connect directly to the ground of the printed circuit board, a very low inductance ground connection is made. A typical value for the total inductance of this connection between a ground pad of the semiconductor die and the ground of the printed circuit board is 0.5 nano henrys. Thus, the total inductance created on the ground connections of the semiconductor die has been reduced from as much as nine nano henrys to as little as 0.5 nano henrys. This lower inductance increases the performance and reliability of the semiconductor die.
Therefore, in order to take advantage of the low inductance on the ground connection resulting from an exposed paddle leadframe based molded package, efforts are made during the leadframe fabrication process to ensure that the bottom surface of the leadframe""s paddle remains free of mold compound in order to make a good solder connection between the bottom surface of the paddle and the printed circuit board. Thus, attempts are made to design the leadframe in such a way that while the leadframe is in the mold cavity of the molding machine, the bottom surface of the leadframe""s paddle is flush with the floor of the mold cavity to prevent mold compound from squeezing under the paddle. However, there is usually some residue of mold compound that penetrates between the bottom surface of the paddle and the floor of the mold cavity. This residue of mold compound left on the bottom surface of the paddle is referred to as xe2x80x9cmold flash.xe2x80x9d
If left on the bottom surface of the paddle, the paddle""s bottom surface will comprise thin lines of solder conductive metal situated between non-conductive mold compound. As such, ground current flow would be restricted to thin lines of the solder conducting metal. This will result in a larger inductance in the ground connection of the semiconductor die. In addition, the flash molding may also result in a mechanically unreliable solder connection between the bottom surface of the paddle and the printed circuit board.
Therefore, exposed paddle based molded packages typically undergo another process, called xe2x80x9cdeflashing,xe2x80x9d to remove the excess mold compound left on the bottom surface of the paddle, which adds to the cost and time expended in the fabrication process. This deflashing process can be done in a chemical bath or by physical abrasion. In the normal process, deflashing removes some flash molding. However, even after deflashing, usually some mold compound still remains on the bottom surface of the exposed paddle.
The problem of unwanted mold compound on the bottom surface of an exposed paddle is aggravated if the leadframe is deformed in such a way as to cause the bottom surface of the paddle not to be flush with the floor of the mold cavity. This deformation of the leadframe may occur, for example, during the wire bonding or die attach phases of the fabrication process or during the handling of the leadframe by the fabrication personnel. When an exposed paddle leadframe which has been deformed is placed in the mold cavity, the bottom surface of the paddle may not be flush with the floor of the mold cavity. As a result, a significant amount of mold compound may penetrate between the bottom surface of the paddle and the floor of the mold cavity.
FIG. 1B illustrates the effects of leadframe deformation discussed above. FIG. 1B shows a cross section view of leadframe 100 after leadframe 100 has been placed inside mold cavity 150 in a molding machine (the molding machine is not shown in any of the Figures). Where the cross section is taken is shown in FIG. 1A by line 1B, also referred to by numeral 134. As stated above, leadframe 100 is a deep downset leadframe. In other words, leadframe 100 has undergone a mechanical forming process whereby paddle 102 is xe2x80x9cset downxe2x80x9d under plane 103 in which the remainder of leadframe 100 resides. The initial xe2x80x9cdownsetxe2x80x9d value, i.e. the initial vertical distance between the plane of paddle 102 and plane 103 is indicated by numeral 156 in FIG. 1B.
As a result of deformation occurring earlier in the fabrication process, the bottom surface of paddle 102 is no longer flush with floor 158. Instead, there is a gap between floor 158 and the bottom surface of paddle 102. As a result, mold compound 136 has penetrated between the bottom surface of paddle 102 and floor 158 of mold cavity 150. The layer of mold compound that will be left on the bottom side of paddle 102 after the molding process is undesirable for the reasons stated above.
Thus, it is seen that there is need in the art for a method of fabricating deep downset leadframe molded packages which prevents mold compound from contacting the bottom surface of the leadframe paddle during the molding process and therefore eliminates the necessity of a deflashing step and allows for a low-inductance and mechanically reliable solder connection between the bottom surface of the paddle of a leadframe and the printed circuit board, resulting in a reduction in the number of rejected molded packages and an increase in the reliability and performance of the non-rejected molded packages.
The present invention is directed to an exposed paddle leadframe for semiconductor die packaging. The present invention overcomes the need in the art for a method of fabricating deep downset leadframe molded packages which prevents mold compound from contacting the bottom surface of the leadframe paddle during the molding process and therefore eliminates the necessity of a deflashing step and allows for a low-inductance and mechanically reliable solder connection between the bottom surface of the paddle of a leadframe and the printed circuit board, resulting in a reduction in the number of rejected molded packages and an increase in the reliability and performance of the non-rejected molded packages.
According to one embodiment of the invention, a leadframe having at least one tab is placed inside a mold cavity. During the molding process, the ceiling of the mold cavity is pressed against the at least one tab, which in turn causes a bottom surface of the leadframe to be pressed firmly against a floor of the mold cavity. When a mold compound is injected into the mold cavity, the mold compound is prevented from reaching under the bottom surface of the leadframe.
In one embodiment, the tabs used in the present invention are etched into the inner tie bars in the leadframe. The tabs are then mechanically formed at a nonzero angle with respect to a plane of a frame of the leadframe.